Electroless nickel plating is a widely utilized plating process which provides a continuous deposit of a nickel metal coating on metallic or non metallic substrates without the need for an external electric plating current. Such a process is described generally as a controlled autocatalytic chemical reduction process for depositing the desired nickel metal and is simply achieved by immersion of the desired substrate into an aqueous plating solution under appropriate electroless plating conditions.
In conducting electroless nickel plating, particularly from a bath which utilizes a hypophosphite as the reducing agent, the bath basically contains a source of nickel cations such as nickel sulfate and a hypophosphite reducing agent such as sodium hypophosphite. The deposition reaction takes place in the bath and generally involves the reduction of a nickel cation to form a nickel metal alloy as a deposit on the desired substrate surface. The reduction reaction is generally represented by the following equation: EQU 3H.sub.2 PO.sub.2.sup.- +Ni.sup.+2 .fwdarw.3/2H.sub.2 .uparw.+H.sup.+ +2HPO.sub.3.sup.-2 +P+Ni.sup.o
It is seen that the electroless reaction produces phosphite ions, hydrogen ions and hydrogen gas; it also produces a counterion of the nickel source compound used, typically a sulfate, SO.sub.4.sup.-2 The nickel and hypophosphite are consumed in the reaction and they, accordingly, must be frequently replenished. In addition, as the hydrogen ions produced in the reaction accumulate they result in a lowering of the pH from the optimum plating ranges. In order to maintain the desired pH range, and in usual practice, a pH adjustor such as a hydroxide or carbonate especially of an alkali metal such as sodium is added frequently during the plating reaction. This significantly increases the monovalent sodium cation concentration of the electroless plating bath.
Additionally, nickel usually in the form of nickel sulfate is added to maintain the optimum nickel concentration thereby increasing the concentration of undesirable sulfate anion. As the reaction continues, the by-products and bath conditions created thereby present problems which adversely affect the desired plating process.
These problems are the buildup of the phosphite anion produced from the oxidation of the hypophosphite reducing agent, the buildup of the anion of the nickel salt employed, typically sulfate, as well as the increased concentration of extraneous cations, especially sodium. This build-up or increase in the concentration of such anions and cations as they accumulate in the bath produces a deleterious effect on the plating reaction and also adversely affects the quality of the plating deposited on the substrate. In particular, the phosphite anion causes an increase in stress of the nickel deposit and shifts the stress from compressive to tensile; this increased stress reduces the corrosion resistance of the nickel deposit. Also, the accumulation of ionic species in the bath degrades the quality of the nickel deposit and makes it unacceptable for such high-level applications as hard discs for computers, as well as CD-ROM and other optical disc storage. Further, the phosphite anions adversely affect the bath by often reacting with and precipitating the nickel cation as nickel phosphite; this slows the rate of deposition of nickel, prevents long lasting baths and results in the bath becoming unsatisfactory and thus terminated at low levels of metal turnover, i.e., the number of times that the original nickel source is replenished. Thus the accumulation of phosphite as well as added alkali metal cations and sulfates prevents the long-term and economical use of the expensive plating solutions and adversely affects the nickel deposit.
These deleterious factors and particularly the build-up of phosphite and sulfate anions have been addressed through use of a variety of treatment methods. These treatments are illustrated in the prior art in such references as G. G. Gawrilov, Chemical Nickel Plating, Portcullis Press, England, 1974; Wei-chi Ying and Robert R. Bonk, Metal Finishing, 85, 23-31, (Dec. 1987); E. W. Anderson and W. A. Neff, Plating and Surface Finishing, 79, 18-26, (March 1992); and K. Parker, Plating and Surface Finishing, 67, 48-52, (March 1980).
Typically these prior art methods have involved treatment of the plating bath solution with calcium or magnesium salts, ferric chloride and anion exchange resins. The use for example of calcium and magnesium results in the generation of large amounts of sludge in the bath caused by the insolubility of the phosphite and sulfate salts of the alkaline earth metals. Ferric chloride addition lowers the pH and introduces iron to the bath.
Mallory, in U.S. Pat. No. 5,338,342 removes by-product phosphite anions by precipitation with lithium hydroxide.